Merge branch 'fix-parallel-builds' into 'master'

Turn the libc and libm rules into 'order-only' prequisites

Closes #127

See merge request redox-os/relibc!158

Makefile

@@ -22,7 +22,7 @@ SRC=\
 
 .PHONY: all clean fmt install libc libm test
 
-all: libc libm
+all: | libc libm
 
 clean:
 	cargo clean